1. Field of the Invention
The present invention generally relates to a pump jack utilizing dual hydraulically operated piston and cylinder assemblies for oscillating the beam about a generally horizontal axis with a control arrangement which positively controls movement of the beam throughout its cycle of operation with the control arrangement including an automatically operated reversing valve operatively associated with the beam and capable of being disengaged from operation by the beam to enable manual operation of the reversing valve together with a hydraulically operated holding valve in each of the hydraulic conduits between the reversing valve and piston and cylinder assembly with each holding valve being controlled from a piston and cylinder assembly interconnecting the conduits so that the hydraulic fluid which operates the dual piston and cylinder assemblies is under positive control thereby providing positive control of the beam throughout its oscillating cycle.
2. Description of the Prior Art
My prior U.S. Pat. Nos. 4,099,447, issued July 11, 1978 and 4,201,115, issued May 6, 1980 disclose hydraulically operated oil well pump jacks with an automatic reversing valve and a cushioning arrangement for cushioning movement of the beam. The reference cited in the above two patents represents some of the prior art.
In addition, the following U.S. patents relate to pump jacks and operating mechanisms therefor:
U.S. Pat. No. 2,169,815--Patterson PA1 U.S. Pat. No. 2,432,735--Downing PA1 U.S. Pat. No. 2,915,919--Mitchell PA1 U.S. Pat. No. 3,264,942--Witt
Of the above listed patents, the Downing patent is probably the most relevant since it discloses a double acting hydraulically operated piston and cylinder assembly for oscillating the walking beam of a pump jack with an automatically operated reversing valve controlling operation of the pump jack.